Fluid flow conditioning

Abstract

There is provided an apparatus (30) and method for conditioning the flow of a mixed phase flow from a supply pipe (101) from a hydrocarbon well. The apparatus (30) comprises an elongate reservoir (11) having a first end for receiving a multi-phase fluid flow from the supply pipe and a second closed end, there being provided a gas outlet (02) from the upper part of the first end, a liquid separation region downstream of the first end, and a liquid outlet (12) from the lower part of the liquid separation region; and a gas-liquid mixer to which the gas and liquid outlets are connected such that the separated gas and liquid may be recombined. The reservoir (11) may accommodate surges of liquid such that the flow rate from the liquid outlet is relatively invariant over time compared to that of the flow received by the apparatus.

Claims

1. An apparatus for conditioning a flow of a mixed phase flow from a supply pipe from a hydrocarbon well, the apparatus comprising: an elongate reservoir having a first end arranged to receive a multi-phase fluid flow directly from the supply pipe and a second closed end, there being provided a gas outlet from an upper part of the first end, a liquid separation region downstream of the first end, and a liquid outlet from a lower part of the liquid separation region; and a gas-liquid mixer to which the gas and liquid outlets are connected such that the separated gas and liquid can be recombined, wherein the reservoir is arranged to accommodate surges of liquid such that the flow rate from the liquid outlet is relatively invariant over time compared to that of the flow received by the apparatus; and wherein the elongate reservoir is a length of the supply pipe from the hydrocarbon well so that the supply pipe itself is used to form the elongate reservoir.

2. An apparatus as claimed in claim 1, wherein the second end is closed by means of a valve located in the supply pipe.

3. An apparatus as claimed in claim 1, wherein the gas-liquid mixer comprises a T-pipe connection.

4. An apparatus as claimed in claim 1, wherein there is provided a supplementary reservoir in series with the elongate reservoir and connected thereto by means of the liquid outlet therefrom.

5. An apparatus as claimed in claim 1, further comprising a plurality of supplementary reservoirs in parallel with each other, wherein each supplementary reservoir is arranged to receive liquid from the elongate reservoir.

6. An apparatus as claimed in claim 5, wherein the apparatus is arranged such that liquid levels in the supplementary reservoirs are balanced.

7. An apparatus as claimed in any of claims 4 to 6 claim 4, wherein the apparatus is arranged such that during use excess liquid in the supplementary reservoirs overflows the supplementary reservoirs and combines with flow from the gas outlet of the elongate reservoir.

8. An apparatus as claimed in claim 1, wherein flow control or restriction devices are provided in the flow paths from the liquid outlet and/or the gas outlet to the gas-liquid mixer to enable the gas/liquid mixture to be adjusted or preset.

9. An apparatus as claimed in claim 1, further comprising an inlet for receiving compressed multi-phase fluid from a compressor and a flow path for returning the compressed fluid to the supply pipe.

10. An apparatus as claimed in claim 9, further comprising a liquid-gas separator in the flow path for returning the compressed fluid to the supply pipe for separating gas from the compressed fluid, and a conduit to permit at least some of the separated gas to be mixed into the supply to the compressor.

11. An apparatus as claimed in claim 10, whereby such wherein recirculation of the separated gas may be controlled to adjust or preset the gas/liquid mixture supplied to the compressor.

12. An apparatus as claimed in claim 1, wherein the gas outlet of the elongate reservoir is arranged to partition flow into parallel flows.

13. An apparatus as claimed in claim 1 in combination with a compressor, wherein the compressor is arranged to receive the conditioned multi-phase fluid from the apparatus, compress the multi-phase fluid, and return the fluid to the apparatus.

14. An apparatus as claimed in claim 13, further comprising a supply line from a hydrocarbon well into which the supply line is connected such that fluid from the supply line flows directly into the reservoir, which forms an extension thereof, and the compressed multi-phase fluid is returned to the supply line at a downstream location.

15. A system for conditioning a flow of a mixed phase flow in a supply pipe from a hydrocarbon well and supplying the mixed phase flow to a multi-phase fluid processing device, the system comprising: an apparatus including: an elongate reservoir having a first end arranged to receive a multi-phase fluid flow directly from the supply pipe and a second closed end, there being provided a gas outlet from an upper part of the first end, a liquid separation region downstream of the first end, and a liquid outlet from a lower part of the liquid separation region; and a gas-liquid mixer to which the gas and liquid outlets are connected such that the separated gas and liquid can be recombined, wherein the reservoir is arranged to accommodate surges of liquid such that the flow rate from the liquid outlet is relatively invariant over time compared to that of the flow received by the apparatus, and wherein the elongate reservoir is a length of the supply pipe from the hydrocarbon well so that the supply pipe itself is used to form the elongate reservoir; and a flow path for receiving processed fluid from the fluid processing device connected to the supply pipe to return the processed fluid thereto, wherein the reservoir is configured to accommodate surges of liquid such that the flow rate from the liquid outlet is relatively invariant over time compared to that of the flow received by the apparatus.

16. A system as claimed in claim 15, wherein the fluid processing device is a compressor.

17. A method of conditioning a flow of a mixed phase flow from a supply pipe from a hydrocarbon well, the method comprising: receiving a multi-phase fluid directly from the supply pipe at a first end of an elongate reservoir, the reservoir having a second closed end, there being provided a gas outlet from an upper part of the first end from which gas flows, a liquid separation region downstream of the first end, and a liquid outlet from a lower part of the liquid separation region from which a mostly-liquid containing fluid flows, wherein the fluids flow to a gas-liquid mixer where the separated gas and liquid are recombined, wherein the reservoir accommodates surges of liquid such that the flow rate from the liquid outlet is relatively invariant over time compared to that of the flow received by the apparatus, and wherein the elongate reservoir is a length of the supply pipe from the hydrocarbon well so that the supply pipe itself is used to form the elongate reservoir.

18. A method as claimed in claim 17, further comprising compressing the multi-phase fluid at a remote fluid processing device and returning the multi-phase fluid to the supply pipe.

19. A method as claimed in claim 18, further comprising separating gas from the compressed fluid and recirculating the gas through the fluid processing device in order to adjust or preset the mixture supplied to the inlet of the fluid processing device.

20. A method as claimed in claim 18, wherein the fluid processing device is a compressor.

Description

(1) Certain preferred embodiments of the present invention will now be described by way of example only with reference to the accompanying drawings, in which:—

(2) FIG. 1 is a plan view of a subsea installation showing a production pipeline and related components;

(3) FIG. 2 is a schematic partially-sectional view of a prior art subsea flow conditioning and compressor system;

(4) FIG. 3 is a schematic elevation showing an overview of a subsea flow conditioning system according to an embodiment of the invention;

(5) FIG. 4 is a schematic sectional view along line A-A of FIG. 3;

(6) FIG. 5 is a more detailed schematic elevation corresponding generally to FIG. 3;

(7) FIG. 6 shows a schematic sectional view of an alternative flow conditioning system according to an embodiment of the invention;

(8) FIG. 7 shows a schematic sectional view of another alternative flow conditioning system according to an embodiment of the invention; and

(9) FIG. 8 shows a schematic elevation of a system comprising a flow conditioning apparatus according to an embodiment of the invention.

(10) Referring first to FIG. 1, there is illustrated a subsea installation 100 showing the relationship between a compressor system and a number of well templates. Specifically, production pipeline 101 receives produced hydrocarbons from wellheads at each of templates 102, 103 and 104 via flexible pipes 105, 106 and 107. In addition, a pig entry unit 108 is provided to allow the pipeline to be pigged when required.

(11) The installation is additionally provided with a subsea compression system that comprises a flow conditioning unit 109 and compressor 110. (The flow conditioning unit replaces a section of pipeline). A jumper 114 connects a controller 112 to the compressor 110. The controller 12 is connected to the shore by cable 113. As is well known in the art, subsea compression systems are provided to allow hydrocarbons to be produced at an acceptable flow rate when the pressure of the underground hydrocarbon reservoir is insufficient, for example after a period of production. The compressed fluid continues onwards at reference 111, eventually to a platform on the surface.

(12) However, where the fluid comprises oil and/or water as well as gas, and is therefore multi-phase, the compression of such fluid is not straightforward. The liquid-phase component is essentially incompressible and is far denser than the gaseous phase. Whilst suitable compressors are able to handle a gaseous flow containing a certain amount of liquids, to facilitate this, the liquid content should be reasonably constant and within an acceptable range. In particular, it is highly undesirable for large “slugs” of liquid to be ingested by the compressor.

(13) A prior art multi-phase flow conditioning and compressor system 10 (disclosed in the applicant's earlier published patent application WO 2009/131462) is shown in FIG. 2. The flow conditioning system catches slugs of liquid and ensures that the liquid content of the fluid is appropriate. It is in the form of a single module which would correspond to the dashed box shown in FIG. 1, but which contains its own compressor 22 instead of remote compressor 110.

(14) In this figure, pipeline 11, 20 (c.f. reference 101 in FIG. 1) supplies produced fluid from the well heads. (When the pressure of the produced hydrocarbon dropped below a certain level, the module was used to replace a section of the original pipeline.)

(15) Thus, with reference to FIG. 2, a section of pipeline 11, 20 comprising a pair of T-pipes (bottom left, not numbered) and valve 13 are shown inserted into the supply pipeline 101. The remainder of the module is connected to the T-pipes.

(16) When the valve 13 is open, fluids may flow along the pipeline 11, 20 and bypass the module. When the valve is closed, the fluid flow is diverted from the pipeline, via further components discussed below, to the compressor, which then ‘sucks’ fluid from the well and eventually returns it to part 20 of the pipeline.

(17) It is important to note that, whilst the figure is schematic, it is in the form of a (partly sectioned) elevation; the relative vertical positions of certain components are important and the compressor 22 is located above the other components.

(18) As noted above, the flow is multi-phase—i.e. liquid and gas—so the use of the compressor is not simply due to the large difference in density between these fluids and the variation in liquid content—i.e. there are “surges” or slugs of liquid that it is difficult for the compressor to handle. For this reason, the module includes a “flow conditioner” 21, which acts as a reservoir to catch and store surges/slugs of liquid and then release them at a more steady rate to the compressor. There is also a gas recirculation system to ensure that the correct gas-liquid mixture is provided to the compressor.

(19) The operation of this prior art system will now be discussed in more detail in the case when valve 13 is closed and the compressor is operational.

(20) All fluid from the well head passes from pipeline 11 via valve 49 to line 61. At T-pipe 62, much of the gas is separated from the fluid because the gas is able to flow vertically (the figure is in the vertical plane) towards the compressor 22, whereas the fluid (which is denser and so has much more momentum) will tend to flow onwards to flow conditioner 21.

(21) The detailed operation of the flow conditioner 21 is described in WO 2009/131462. For the present purposes, it is sufficient to note that it acts as a reservoir to catch slugs of liquid and hold the liquid flowing into it and then to release it at a relatively steady rate via line 66 to be mixed back into the line from T-pipe 62.

(22) The system may be balanced (i.e. the relative flows adjusted as desired) by means of flow restrictor 63, which restricts the flow of gas from T-pipe 62 to the mixing point.

(23) The mixed fluids then pass through multi-phase flow meter 46 to compressor 22. The flow meter 46 is used in the control of the compressor, which compresses the fluid.

(24) On the output side of the compressor, separator 47 separates gas, which may be re-circulated back through the compressor in order to provide an optimum gas/liquid mixture. The separated gas flows via valve 19, which controls the amount of gas that is re-circulated.

(25) The remaining fluid is then returned to part 20 of the supply pipe at a T-pipe via valve 51.

(26) In a practical system, the main supply pipe 11, 20, 101 is typically 30″ in diameter. The take-off pipe to the compressor is 18″ and the discharge pipe from the compressor may be 14″ (since the compressed fluid has a smaller volume).

(27) For low liquid velocity, it is relatively easy to separate liquid from gas since there is stratified flow (liquid at the bottom of the pipe). At high liquid flow velocities the gas and liquid are much more intermixed, e.g. as disbursed bubbly flow. The flow velocity will be lower in the larger diameter pipes—in the 30″ pipe it will be either stratified (smooth or wavy) or “slug” flow, in the 18″ pipe it will be annular (mixed up) flow due to the higher velocity.

(28) These flow characteristics have been taken advantage of in the embodiment of the invention that is shown in the remaining figures.

(29) Returning firstly to FIG. 1, the embodiment comprises a flow control unit or module 30 (c.f. reference 109 of FIG. 1) that is used in combination with a compressor 110. In the illustrated embodiment, the compressor is remotely located, perhaps some hundreds of metres distant, elsewhere on the seabed. However, the compressor and flow conditioning unit may be formed as a single module, as in the prior art system.

(30) The module may be connected into the pipeline by means of Mogrip (proprietary) connectors, which are well-known for interconnecting connecting two pieces of large-diameter pipe in the oil and gas field.

(31) An important principle underlying its operation may be seen from FIGS. 4 and 5. The stratified or slug flow in the large-diameter main supply pipeline 11 (c.f. 101 of FIG. 1) means that it is well-suited for separating liquid from gas. Accordingly, the shaded region is utilised as a flow-conditioner.

(32) As in the prior art system, a pair of T-pipes 02 and 25 are provided in the supply pipe 11 to allow for fluid flow to and from the compressor respectively. In practice, this is done by replacing a section of the pipe with that shown in the dashed box. The central valve 26 corresponds to valve 13 of the prior art system, though it is located so as to provide a substantial region of pipeline 11 downstream of it. This is provided with a pair of downwardly-directed T-pipes 12.

(33) Thus, the liquid will flow into the shaded region and so, unlike the prior art system, the flow from T-pipe 02 is mostly gas. Since the fluid in the shaded part of the supply pipe has a relatively low flow velocity, the liquid will tend to separate out naturally and settle at the bottom of the pipe, as may be seen from FIG. 5. The downwardly-directed T-pipes 12 then provide a flow path for this liquid to the mixing point 06 where the liquid is mixed back into the flow of gas to the compressor.

(34) Thus, this arrangement avoids the need for a separate flow conditioner, as in the prior art, though supplementary separation devices may be provided (see below). This in turn allows for a much smaller system and in the case of a modular design including a compressor in the module, for a reduced height module since the compressor can be located much closer to the supply pipe. This allows for a significant reduction in the size of that module.

(35) FIG. 5 shows the module 30 of the embodiment in more detail. As shown, pipeline 11 has a first end 01 for connection to the main pipeline from the production templates and a second end 27 for connection to the pipeline leading to the production platform at the surface. As already described, T-pipe 02 provides a connection to allow mostly-gas fluid to flow vertically therefrom, T-pipe 25 provides a further connection to allow compressed fluid to return to the pipeline, and T-pipe 12 provides a downward path for the mostly-liquid flow from the region of pipe 11 shown shaded in FIG. 4. (For simplicity the following comments will refer to liquid and gas flows respectively but it should be born in mind that both of these are multi-phase flows, i.e. they are mostly-liquid and mostly-gas respectively.)

(36) From T-pipe 02, the gas flows along pipe section 03, past T-pipe 04, to further pipe section 05 where it meets T-pipe 06. An alternative gas flow metering device 09′ may be disposed downstream of T-pipe 04 and upstream of pipe section 05. An alternative cooler 08′ may be positioned upstream of T-pipe 06 and downstream of pipe section 05. Here the separated liquid (see below) is mixed back into the gas flow. From there the combined fluids flow via further pipe section 07 to optional cooler 08 and then via multi-phase flow meter 09 to outlet 10 leading to the remote compressor 110 (FIG. 1). The multi-phase flow meter 09 may be optional. If the cooler 08′ is used then liquid from the supplementary reservoir 14 may have a higher temperature than the gas at the T-pipe 06 where mixing occurs.

(37) Returning to the flow into pipeline 11, as previously described, the liquid settles to the bottom of the shaded part (FIG. 4) of pipeline 11, which it then leaves under gravity via T-pipe 12 and flows via flow restrictor 15 to T-pipe 06 where it is mixed into the gas flow in pipe section 05, as discussed above. An alternative liquid flow metering device 15′ may be included to control the flow. The flow restrictor 15 may be used to adjust the system to obtain the desired mixture of liquid and gas in pipe section 07. The flow restrictor 15 may be used to adjust the system flow resistance to obtain the desired mixture of liquid and gas in pipe section 07. It could also provide the functionality of liquid flow metering device 15′, depending on the type of flow restrictor used. The use of the flow metering devices 09′ and 15′ may provide a more accurate flow metering compared to device 09, however the actual selected combination may depend on the application.

(38) It will be noted that FIG. 5 shows an optional additional separation volume 14, which may be provided to augment the separation that occurs in the separator part of pipeline 11. As shown, it is in series with the separator part of the pipeline and connected to it by means of pipe section 13. Alternatively, the length of the separator section of pipeline 11 may be extended by changing the relative locations of the T-pipe 02 and valve 26.

(39) Referring to the upper part of the figure, the return (compressed) flow from the compressor enters the module via pipe section 20. The remaining steps are similar to the prior system in that the multi-phase flow passes to separator 21 from which a portion of the gas flows via pipe section 22 and control valve 23 to T-pipe 04 where it is returned to the input gas flow. This provides controlled recirculation in order to balance the oil-gas mixture flowing to the compressor. The mixed phase flow (except the re-circulated gas portion) is returned to the pipeline at T-pipe 25 for onward transmission.

(40) The system is balanced by a combination of adjustments to the flow restrictor 15, which changes the flow of (mostly) liquid into the flow to the compressor and valve 23, which adjusts the amount of re-circulated gas flowing to it.

(41) FIG. 6 shows a sectional view of an apparatus for conditioning flow comprising an additional separation volume or supplementary reservoir 14. The supplementary reservoir 14 is arranged to receive flow from a pipe section 13, which itself receives liquid flow from the liquid outlet of the elongate reservoir 11. The pipe section 13 is shown in FIG. 6 connecting to the supplementary reservoir at a point half-way up the pipe, but it will be appreciated that the pipe section 13 may communicate with the supplementary reservoir at any suitable position. The supplementary reservoir 14 has a gas outlet in an upper section which permits gas flow via pipe section 120 to combine with gas from the elongate reservoir 11 in pipe 133. Liquid may leave the supplementary reservoir 14 via a liquid outlet in a lower portion into pipe 121. That liquid is combined with gas flow from T-pipe 03 via pipe 123 and into pipe 134. Valves 15 and 15′ are arranged to control the flow of liquid from the supplementary reservoir into pipes 133 and 134 via pipes 135 and 135′ respectively. Flow from pipe 133 enters pipe 07, and flow from pipe 134 enters pipe 07′. The pipes 07 and 07′ may be arranged to transport flow of liquid and gas, and may or may not be connected downstream to a compressor or the like. They may serve to reduce the pressure drop in the system.

(42) The apparatus of FIG. 6 also comprises a T-pipe 130 connected to the T-pipe 03 which serves as the gas outlet of the elongate reservoir 11. As such, gas from the elongate reservoir 11 is split into two parallel flows in pipes 133 and 134. The gas and liquid from the supplementary reservoir 14 is combined with gas flow in the pipes 133 and 134 as described above via valves 15 and 15′ or other suitable flow restrictors. Flow restrictors or the like may also be used on pipes 133 and 134 to provide greater control over the flows therein and allow balanced mixing of gas/liquid and water to control hydrate occurrence.

(43) A second supplementary reservoir 14′ may be included, as well as a pipe section 136 arranged to receive flow from the elongate reservoir 11 and pass that flow to the second supplementary reservoir 14′. The liquid from the elongate reservoir 11 may be evenly distributed to the first and second supplementary reservoirs 14 and 14′ e.g. by gravity. Pipes 131 and 121′ may also be provided to permit combination of gas and liquid respectively with the flow from T-pipe 130 in pipes 133 and 134. The arrangement of the second supplementary reservoir 14′ may be the same as for that of the supplementary reservoir 14, albeit in relation to pipe 134 instead of pipe 133.

(44) The arrangement shown in FIG. 6 may allow liquid levels in the first and second supplementary reservoirs 14 and 14′ to be balanced so that an equal liquid injection rate is achieved. It may also allow via valves 15 and 15′ the liquid to be evenly distributed into the two gas pipes 133 and 134, which even distribution may ensure evenly distributed glycol and/or anti-hydrate fluid for the downstream gas-condensate transportation system. Flow restrictors or the like may be provided on pipes 133 and 134 to provide control over the flows therein and hence may be used to exercise further control over the combination of gas and liquid. Finally the produced gas and liquid is distributed evenly into pipes 07 and 07′.

(45) FIG. 7 shows a flow conditioning apparatus comprising a supplementary reservoir 14 similar to that of FIG. 6. However, in FIG. 7 the gas flow from the elongate reservoir 11 is not divided into two flows and all flows along T-pipe 03. The arrangement of FIG. 7 may be used to accommodate large volumes of liquid and gas from the wellhead.

(46) Additional supplementary reservoirs 14′ and 14″ may be provided in parallel with the supplementary reservoir 14 and may be arranged to receive liquid flow from the elongate reservoir. Each supplementary reservoir 14, 14′ and 14″ may have their gas outlets connected by pipe sections 122, 122′ and 122″ respectively to a pipe 120 so that gas flow therefrom combines with gas flow from the elongate reservoir in T-pipe 03. The liquid outlets of each supplementary reservoir 14, 14′ and 14″ may also be arranged to provide liquid to the pipe 123 via pipe sections 121, 121′ and 121″ respectively such that liquid combines with the flow in T-pipe 03. A valve 15 may be provided to control the flow rate of liquid to the pipe 07 e.g. for transport to the compressor.

(47) According to the arrangement shown in FIG. 7, the liquid levels in each of the supplementary reservoirs 14, 14′ and 14″ may be evenly balanced as the pipes therebetween may be open. Thus, greater liquid volume may be accommodated in the apparatus. If the valve 15 is configured to permit less liquid through than is flowing from the supplementary reservoir 14, the excess liquid can flow up pipes 121′ and 121″ to fill the reservoirs 14′ and 14″ respectively. Gas can exit those supplementary reservoirs 14′, 14″ via pipes 122′ and 122″ to permit them to fill. In the event that a large amount of liquid enters the system and fills all of the supplementary reservoirs, excess liquid can flow from the supplementary reservoirs 14, 14′ and 14″ through pipe 120 to combine with flow in pipe 03, thereby bypassing the valve 15.

(48) Each supplementary reservoir 14, 14′, and 14″ may be substantially the same as the elongate reservoir 11 and therefore may be formed of a section of hydrocarbon supply pipeline as described above. Each supplementary reservoir 14, 14′ and 14″ may be closed at both ends to provide a containing volume.

(49) The additional supplementary reservoirs 14′, 14″ and so on may be added to the apparatus as needed, and may for example be installed in the apparatus after the apparatus itself has been installed. Any suitable number of supplementary reservoirs can added as needed. Therefore, increases in the total amount of fluid flow from the wellhead can be accommodated by increasing the capacity of the apparatus to accommodate surges of liquid. Flow metering devices may also be applied here as described in FIG. 5. Such devices can be used to adjust the restriction 15 together with a level measurement device between the elongate reservoir 11 and the pipe 123.

(50) FIG. 8 shows a schematic arrangement wherein different mechanisms are provided to prevent large reserves of liquid in the supplementary reservoirs from flooding the compressor. In some cases, accumulated liquid in the apparatus may be drained by gravity or forced by gas flow into the compressor inlet flow line e.g. when the compressor is shut down. In such cases, upon start-up a large volume of liquid may be pushed (e.g. by gas) into the compressor having detrimental effects. This issue may be avoided using the arrangement shown in FIG. 8.

(51) Flooding of the compressor in such a situation may be avoided in three ways using the arrangement of FIG. 8. Firstly, the flow restriction device 15 may be used to close or restrict flow of liquid from the supplementary reservoir 14 into the pipe 07 to the suction line 10 of the compressor.

(52) Secondly, the reserve liquid may be drained under gravity to a location 34 upstream of the elongate reservoir 11 via a restriction 31 and a pipe 36. Although FIG. 8 shows the location 34 above the supplementary reservoir 14, the apparatus may be arranged such that the location 34 is below the supplementary reservoir 14 but upstream of the elongate reservoir 11.

(53) Thirdly, the liquid may be drained (e.g. under gravity) to the location 35 of the exit pipe 20 of the compressor via valve 32 so as to bypass the compressor entirely. The valve 32 may be an anti-surge valve and may normally open during a shut down. Alternatively, the pipe joining pipes 10 and 20 may not include a valve (e.g. valve 32) and may simply be narrow so that liquid can pass through it at a predetermined rate.

(54) The compressor itself is controlled by means of a conventional variable speed drive (VSD), with the multi-phase flow meter 09 providing a control input.

(55) Even with the use of the flow conditioner when properly adjusted, there will still be variations in the liquid content of the fluid that is fed to the compressor with time. For a given speed, more liquid requires a higher torque and hence a greater supply of electrical current. The conventional approach is to use the VSD in constant speed mode, which means that the mass-flow from it will vary over time and consequently significantly varying amounts of electrical power are drawn.

(56) The control system of the embodiment involves using the VSD in constant torque mode—i.e. the VSD is instructed to keep the torque at a single value and to allow the speed to drop when more liquid is present—at least when fluctuations in torque exceed a predetermined threshold of 5%. Earlier published application WO 2016/206761 describes constant-speed control (using measurement of the current drawn by a compressor to determine the liquid flow through it). The controller 112 (see FIG. 1) comprises the VSD controller for the compressor and a processor to receive inputs from the multi-phase flow meter controller and/or act upon information about liquid content—e.g. the presence of liquid slugs—upstream of it. The controller 112 also receives and/or acts upon information about the current drawn by the compressor, and/or provides output signals to control the valves and flow restrictors. The controller 112 thus monitors fluid content in and/or upstream of the apparatus via the flow measuring device and compressor current draw in order to control the compressor so as to operate in a constant torque mode when the compressor shaft torque fluctuates too much.

(57) In this way, the controller 112 prevents compressor shaft torque variations due to liquid surging, etc.

(58) As a result, at least where the liquid content causes relatively high shaft torque fluctuations of greater than 5%, the controller will switch control modes of the compressor to maintain a substantially constant torque level. If the liquid content has increased, the speed of the compressor will be reduced in order to maintain a constant torque. Also, since a greater liquid content will result in the compressor generating a higher output pressure, this control regime has the advantage of stabilising the compressor pressure ratio over time.

(59) If the torque does not fluctuate by more than 5%, the controller 112 is configured to control the compressor 110 not to operate in a constant torque mode, and may instead operate in e.g. a constant speed mode. Thus, when torque fluctuations exceed 5%, the controller is configured to switch from its current mode (e.g. constant speed) to the constant torque mode. The controller may be configured to switch to the constant torque mode when the fluctuation of torque is greater than 10% in about one second. The controller may be further configured to switch out of a constant torque mode of operation when another predetermined operating condition is met, or when it is otherwise instructed to do so. The controller 112 may be monitored or controlled remotely (e.g. from shore) via the cable 113.

(60) The fluctuation of the torque may be a change in torque per unit time of the order of a second or so. That is, the torque fluctuation may be change in the total amount of torque in a predetermined time period. Therefore, the controller may switch into a constant torque mode when the torque changes by more than a predetermined threshold amount at greater than a predetermined rate. The change in torque may be a change in the total amount of torque in a given time.

(61) Although the controller 112 is depicted as connected to the compressor via jumper 114, it will be appreciated that it may be integral to the compressor, or the VSD may be integral to the compressor with other components of the controller not integral thereto and disposed remotely.